1. Field of the Invention
The present invention relates, generally, to communication network management and, in one embodiment, to a method and apparatus for generating and utilizing a relative future activity indicator for selecting the source of received communications.
2. Description of Related Art
Cellular communication networks are rapidly becoming a primary infrastructure for enabling communication in today""s society. In addition to providing a means for voice communications such as personal or business telephone calls, cellular communication networks are now being used for transmitting data. As demand for cellular communications has increased, cellular communication networks are becoming increasingly prevalent and are providing coverage over larger areas to meet consumer demand.
FIG. 1 illustrates an example system environment including a mobile station (MS) 10 capable of maintaining a connection 30 with a cellular communication network 22 as the MS 10 roves through a geographic area served by the cellular communication network 22. It should be understood that a connection, as referred to herein, includes, but is not limited to, voice, multimedia video or audio streaming, packet switched data and circuit switched data connections, short message sequences or data bursts, and paging. The cellular communication network 22 includes a first base station (BS) 12 communicating over sectors 14 and 16, and a second BS 18 communicating over sector 20. A BS is typically comprised of multiple sectors, usually three. Each BS includes a separate transmitter and antenna (transceiver) for each sector, pointed in the direction of the sector. Because a BS can be omni or sectorized, it should be understood that the terms BS and sector may be used interchangeably herein. Furthermore, when referring to access to a network via a network access point, the terms BS, sector, and network may be used interchangeably herein. The BSs are connected to network infrastructure entities including BS controllers (BSC) 24 that may control a cell cluster 26, and communicate with a mobile switching center (MSC) 28. It should be understood that the MS 10, BSs and one or more of these network infrastructure entities may contain one or more processors for controlling communications between the MS 10 and the network 22. The processors include memory and other peripheral devices well understood by those skilled in the art.
In the course of roving, MS 10 travels from position A to position B to position C and will, as a matter of course, experience variations in signal strength and signal quality of the communication link associated with the BS(s) that it is in contact with. Signal strength and quality can be especially undependable near the edges of the sectors, such as when the MS 10 transitions from the area defined by the dotted line of sector 14 to the area defined by the dotted line of sector 16, or from sector 16 to sector 20.
FIG. 2 illustrates an exemplary communication link 30 between a MS 10 and a BS 12. Communications from the BS 12 to the MS 10 are called the forward link, and communications from the MS 10 to the BS 12 are called the reverse link. The forward and reverse links utilize a number of forward and reverse channels. For example, the BS 12 communicates with the MSs using a plurality of forward common channels or links which may include, but are not limited to, one or more pilot channels, a sync channel, and one or more paging channels, discussed in greater detail below. These channels are referred to as common channels because the BS 12 may communicate those channels to all MSs in the network. Generally, these common channels are not used to carry data, but are used to broadcast and deliver common information.
Each sector within BS 12 broadcasts a pilot channel that identifies that sector and is simple for a MS 10 to decode. Both sectors and pilot channels are distinguished by pseudo-noise (PN) offsets. The word xe2x80x9cpilotxe2x80x9dcan be used almost interchangeably with the term sector, because a pilot channel identifies a sector. The pilot channel implicitly provides timing information to the MS, and is also used for coherent demodulation, but it otherwise typically does not contain any data. When a MS is first powered up, it begins searching for a pilot channel. When a MS acquires (is able to demodulate) a pilot channel, the timing information implicit in the pilot channel allows the MS to quickly and easily demodulate a sync channel being transmitted by the network.
Because the sync channel contains more detailed timing information, once the MS acquires the sync channel, the MS is then able to acquire a paging channel being transmitted by the same BS that is transmitting the pilot channel. That BS is known as the active BS. When a cellular network is attempting to initiate communications with a MS through a particular BS, a xe2x80x9cpagexe2x80x9dis transmitted to that MS on the paging channel of that BS. Thus, once the MS is able to demodulate the paging channel of a particular BS, the MS may then monitor that paging channel while the MS is idle and waiting for incoming connections or an incoming message. In general, each BS may utilize one pilot channel, one sync channel and one paging channel that are common for all MSs to receive. However, because there are practical limitations on the number of MSs that can be simultaneously paged using one paging channel, some BSs may employ multiple paging channels.
In addition to the forward common channels described above, the BS 12 communicates with individual MSs using a plurality of forward dedicated channels or links which may include, but are not limited to, multiple traffic channels, multiple supplemental channels, and multiple access channels and control channels. These channels are referred to as dedicated channels because the BS communicates the channels to a specific MS 10, and the channels may carry data.
The reverse channels or links may include an access channel and one or more reverse traffic channels and control channels. After a MS receives an incoming page from a BS, the MS will initiate a connection setup using, in part, an access channel.
The previously described channels may employ different coding schemes. In time division multiple access (TDMA), multiple channels may be communicated at a particular frequency within a certain time window by sending them at different times within that window. Thus, for example, channel X may use one set of time slots while channel Y may use a different set of time slots. In frequency division multiple access (FDMA), multiple channels may be communicated at a particular time within a certain frequency window by sending them at different frequencies within that window.
Code division multiple access (CDMA) is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. It allows a number of MSs to communicate with one or more BSs in neighboring cell sites, simultaneously using the same frequency. In CDMA, given a space of frequency and time, each channel is assigned a particular orthogonal code such as a Walsh code or a quasi-orthogonal function (QOF). In direct sequence CDMA, the data from each channel is coded using Walsh codes or QOFs and then combined into a composite signal. This composite signal is spread over a wide frequency range at a particular time.
When this composite signal is de-spread using the same code used to spread the original data, the original data may be extracted. This recovery of the original data is possible because Walsh codes and QOFs create coded data that, when combined, don""t interfere with each other, so that the data can be separated out at a later point in time to recover the information on the various channels. In other words, when two coded sequences of data are added together to produce a third sequence, by correlating that third sequence with the original codes, the original sequences can be recovered. When demodulating with a particular code, knowledge of the other codes is not necessary.
In CDMA systems, signals can be received in the presence of high levels of narrow-band or wide-band interference. The practical limit of signal reception depends on the channel conditions and interference level. Types of interference include those generated when the signal is propagated through a multi-path channel, signals transmitted to and from other users in the same or other cell sites, as well as self-interference or noise generated at the device or MS.
Typical CDMA wireless communication systems are fully described by the following standards, all of which are published by the TELECOMMUNICATIONS INDUSTRY ASSOCIATION, Standards and Technology Department, 2500 Wilson Blvd., Arlington, Va. 22201, and all of which are herein incorporated by reference: TIA/EIA-95A, published in 1993; TIA/EIA-95B, published Feb. 1, 1999; TIA/EIA/IS-2000, Volumes 1-5, Release A, published Mar. 1, 2000; TIA/EIA-98D, published Jun. 1, 2001; and WCDMA standards 3GPP TS 25.214 V4.2.0 published September 2001, TS25.401 V5.1.0 published September 2001, TS 25.331 V4.2.0 published Oct. 8, 2001, and TR 25.922 V4.1.0 published Oct. 2, 2001.
As illustrated in FIG. 3, a channel 32 may be broken up into superframes or slots 42, typically of 80 millisecond duration. Each slot may be divided into three phases 44. These phases are numbered 0, 1 and 2 in FIG. 3. Coincident with the timing of the phases are four frames 34. These four frames are aligned with the three phases at the superframe boundaries. Each frame 34 is therefore typically 20 milliseconds long. Other frame sizes such as 5 ms, 10 ms and multiples of 20 ms can also be used. A message is typically comprised of one or more frames.
Within a superframe, preambles of various lengths can be transmitted prior to transmission of the frames, for example, in the case of reverse access channels and reverse common control channels. It should be understood that the content of the frames 34 can differ. One frame may contain a header 36, signaling 38 and data 40 multiplexed on different code channels, another may contain only signaling, and yet another may contain only data. Each frame 34 may also have a different data rate, which can be changed on a frame-by-frame basis. In some example communication standards, there are four rates: full, one-half, one-fourth and one-eighth. Thus, for example, with no voice activity, information may be transmitted at a one-eighth frame rate, which would be beneficial because less power or bandwidth is required to communicate information at a slower rate. The network capacity can be increased as the interference is reduced.
In response to increased demand for packet data communications at increasingly higher data rates, High Data Rate (HDR) standards have been developed. The 1xc3x97EV-DO(HDR) standard (1xc3x97 the bandwidth of IS-95, EVolution, Data Only, High Data Rate) supports only data transmissions and is incompatible with the IS-95 and IS-2000 standards mentioned above. These shortcomings are being addressed by the development of the 1xc3x97EV-DV standard (1xc3x97 the bandwidth of IS-95, EVolution, Data and Voice), which will support both data and voice and be compatible with IS-95 and IS-2000.
FIG. 4 illustrates an example system environment 46 comprising three BSs A, B, and C and a MS 10 within the coverage area for all of them. In the example of FIG. 4, the communication system is 1xc3x97EV-DV so both voice and data are capable of being transmitted. Thus, the MS 10 may receive voice communications, control and overhead information 48 from BS A while simultaneously maintaining a packet data communication link 50 with BS C for a web browsing session.
Because the MS 10 is within the coverage area of BSs A, B, and C, the MS may maintain an active/eligible set of A, B and C for voice communications, control and overhead. If the MS 10 moves outside the range of one of the BSs, the MS 10 may request to modify its active/eligible set accordingly. If the MS maintains simultaneous voice communication links with more than one BS, the MS would be in soft or softer handoff.
On the other hand, a MS will typically not maintain simultaneous packet data communication links with multiple BSs. A MS typically selects, through fast cell selection, one (but only one) BS (network access point) in its active/eligible set for establishing a packet data communication link. Fast cell selection is a feature of communication systems such as those described in the cdma 1xc3x97EV-DO(HDR) and 1xc3x97EV-DV standards. To select the best BS/sector from which to receive packet data transmissions, the MS first determines the strength of the signal from the BSs (e.g., the carrier to interference ratio C/I). The MS may also estimate the future signal strength of any particular channel being received from a BS based on current and historical measurements maintained by the MS. Once a BS is selected by the MS, an identification of the BS is transmitted to the network. From this transmission, the network then identifies all MSs that have selected the same BS, and chooses the MS that is best receiving the BS. This methodology is related to a proportionally fair scheduling system wherein the user who best receives the data is given priority in a manner that also considers the latency impact to each user.
As noted above, the MS may estimate the future signal strength of any particular channel being received from a BS based on current and historical measurements maintained by the MS. However, the MS does not have any indication of future interference levels that may exist due to other transmissions or inter-cell interference. Such information may affect the quality of the transmission from the channel. Therefore, during development of the HDR standards, a forward activity bit (FAB) was proposed to augment the fast cell selection process. In this proposal, a BS would transmit a FAB to a MS, providing some indication of expected future transmissions on the forward traffic channel so that the MS can better decide which BS will be best for receiving the forward traffic channel. The FAB would indicate either some expected future activity or no expected future activity. No indication of how much activity would be provided. An improvement to the original FAB was later proposed in which an additional bit would be provided that indicated either some or no activity for odd and even slots.
One drawback to the proposed FAB schemes is that the FABs do not provide any indication of the direction or magnitude of the predicted future activity relative to current activity levels (e.g., higher or lower, and by how much). Such information would enable the MS to even more accurately predict which BS (network access point) will be best to connect/handoff to (receive forward frames from).
Another drawback in systems that may transmit to more than one user is that a distinction cannot be made between a situation in which the full forward link bandwidth is in use and one in which some bandwidth is available for additional traffic even though there is activity.
Therefore, a need exists for a method and apparatus for generating and utilizing a relative future activity indicator that provides an indication of the direction and magnitude of the predicted future activity relative to current activity levels for the purpose of enhancing the selection of the source of received communications.
In fast cell selection, the MS selects one or more potential network access points to receive from (i.e. the forward link active/eligible set). To select the best BS/sector from which to receive transmissions, the MS can determine the strength of the signal from the BS (e.g., the carrier to interference ratio C/I). The MS may also estimate the future signal strength of any particular channel being received from a BS based on current and historical measurements maintained by the MS. Once a BS is selected by the MS, an identification of the BS is transmitted to the network. From this transmission, the network then identifies all MSs that have selected the same BS, and chooses the MS that is best receiving the BS.
Embodiments of the present invention provide a future activity indicator including the direction and magnitude of predicted future activity relative to current activity levels (e.g., higher or lower, and by how much) to enable the MS to even more accurately predict which BS (network access point) will be best to connect/handoff to (receive forward frames from). The future activity indicator may comprise one or more future activity indicator (FAI) bits.
To determine and transmit FAI bits, the BS first determines its current actual activity or interference level A(t), and then may determine its resource assignments, connections, scheduling and expected reconnects (in other words, the expectation of future transmissions or activity). The BS then predicts future activity from this determined expectation of future activity. The future predicted activity is designated Axe2x80x2(t+1), where the prime indicates that the future activity is predicted. The BS then computes the predicted future change in activity xcex94Axe2x80x2(t+1) as the future predicted activity Axe2x80x2(t+1) minus the current actual activity A(t). The predicted future change in activity xcex94Axe2x80x2(t+1) is translated into FAI bits using a mapping into a table or a calculation or formula, and these FAI bits are transmitted to the MS on the forward link.
The MS receives the FAI bits corresponding to xcex94Axe2x80x2(t+1) from one or more BSs/sectors, decodes them and stores them. Note that in one embodiment the FAI bits may be interpreted as an indication of the expected change in the pilot strength PSs(t) for a BS/sector, i.e. xcex94PSxe2x80x2(t+1). The MS also measures the pilot strengths PSxe2x80x2s(t) of sectors eligible to be serving sectors. The measured pilot strength of each BS/sector is then biased using relative pilot strength bias values corresponding to the received FAI bits. Such biasing may be accomplished, for example, by adding the expected change in pilot strength xcex94PSxe2x80x2s(t+1) to the pilot strength PSs(t) to compute a predicted future pilot strength PSxe2x80x2s(t+1)=PSs(t)+xcex94PSxe2x80x2s(t+1). The best eligible sector is selected as the desired serving sector, and a corresponding indication of the best eligible sector is then sent to the BS over the reverse link. This message is a request by the MS that future transmissions originate from the selected BS/sector, but the network is typically the final arbiter of the cell selection, and may override this request and select another BS.